SECTION 3 - BRUSH THEORY AND CHARACTERISTICS
Question 3:1 What is "Commutation"?
In DC Machines, in addition to conducting current to and from the armature the brush assists in carrying out a difficult switching operation, the reversal of the current flow in the armature coils. This is called "commutation".
As the armature rotates, the current in any particular coil of the armature winding must change direction as it passes from one branch or circuit path to the next which has the opposite polarity. The position this occurs is directly under the brush.
The current cannot instantaneously change direction. As a result of armature inductance and other induced voltages this process has a time constant, additionally it does not usually take place in a linear way.
As a brush begins to short out two commutator segments connected to a coil which is still carrying current, as it has not yet reached zero, the brush provides an alternative path for the current to take. Depending mainly on the resistance, and contact drop of the brush, a proportion of current will flow across the brush face from segment to segment.
Once the current in the coils reaches zero there is no current across the brush face between the segments connected to that coil. This is called the neutral position or neutral zone.
There can be a similar affect as the current tries to build up in the opposite polarity. There is a tendency for current to flow from segment to segment across the brush face in addition to passing through the armature coil connected to those segments.
This is why arcing on brushes generally occurs at the leading or trailing edges of brushes whereas the middle of the brush has less electrical stress on it having only to conduct current to or from the commutator segment from the connected supply.
The machine design and set up determines the level of current that may potentially flow in the brush face. Some machines are very well designed, others may present the brush supplier with a real challenge to control the current and arcing to a satisfactory level.
Another factor is that rotation of the armature causes the field flux to become distorted and therefore the neutral position will be altered to some degree. Most larger machines have interpoles fitted which are designed to counteract the field flux distortion and maintain the magnetic stability of the neutral zone.
Question 3:2 What Part Does Resistivity Play In A Brush?
The information has been drawn from experience of Morganite Taiwan Limited Application Specialists combined with a wide range of published information from major carbon supplier and motor manufacture. However, the material can not be downloaded, copied or used without written permission.
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